Method for correcting ignition advance of an internal combustion

ABSTRACT

The present invention is a method of ignition advance correction of an internal-combustion engine according to the composition variation of the natural gas contained in a tank and supplying the engine. A methane number relative to the natural gas filling the tank is determined and an ignition advance correction value is deduced from the number. According to the invention, a correlation is established between the methane number described above, an air-fuel ratio R for a stoichiometric combustion and the gas flow variation required to reach a reference working point of the engine after the composition variation of this gas.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the field of spark-ignition engines fedwith natural gas or with another power gas.

2. Description of the Prior Art

A problem inherent in spark-ignition engines lies in the wide range ofnatural gases available through the sales networks. In fact, greatvariations in the composition of the gases that are marketed have beenobserved. These gases can either correspond to the native gas or consistof a mixture of identified and known gases.

Therefore, although the composition of the gases available on thenetworks necessarily meets the standards of the countries where they aremarketed, there are however great differences between two distributingpoints in one country; there are also differences between thedistributions of different countries.

Furthermore, the composition of the gas delivered by a station can varyaccording to the period of the year; this is due to the supplyvariations of the stations.

Vehicles running on natural gas (NG) must however be supplied anywhereand anytime without any engine running problems.

These problems are already mentioned for example in U.S. Pat. No.5,537,854 which describes a piezoelectric device allowing knockphenomena to be linked with the acoustic properties of the fuel (NG).

It is therefore necessary to know and/or to estimate the composition ofthe natural gas with which a given engine is supplied because thecomposition influences the running conditions of the engine, notably asregarding engine knock.

In fact, it is well-known that engine knock is correlated with themethane number (IM) of the gas, which itself depends on the compositionof the gas, which directly influences the appearance of engine knock.Thus, if the methane number of a natural gas is known with sufficientprecision, it will be possible, notably from engine maps, to deduce acorrective ignition advance value allowing the optimum ignition advanceto be used in order to prevent knock phenomena or to allow optimumcombustion adjustment.

A suitable engine tuning can thus be found.

Extensive research has already been carried out into natural gascharacterization. Complex analysis systems have been described, forexample in U.S. Pat. No. 5,333,591. According to this prior art, adevice which analyzes of the composition of the natural gas, associatedwith a control unit, allows the engine to be tuned. This analysis isbased here on the thermal conductivity of the gas.

It is also possible to use a chromatographic analysis to determine thecomposition of the natural gas. However, this solution is expensive andquite difficult to apply in a mobile and limited environment.

SUMMARY OF THE INVENTION

The present invention provides optimum engine running conditionsregarding ignition advance, without risk of knocking. The inventionimproves the combustion while ensuring notably an optimum combustionvelocity. The appearance of engine knock is advantageously prevented bydecreasing the ignition advance as soon as required. Furthermore, thepresent invention provides control of ignition advance without theengine having to run in the knock zones, notably the initial knockzones.

The present invention provides, from at least one working point of theengine or reference point, a determination of the variation of aquantity linked with the variation of the flow of power gas entering theengine, these variations resulting from the change in thecharacteristics of the power gas, and from the variation of thisquantity, determining the ignition advance correction to be applied atvarious working points of the engine.

The quantity linked with the variation of the flow of power gas can bethe flow of gas itself It is possible, from this variation and from theair/power gas ratio of the mixture supplying the engine, to determinethe stoichiometric air mass quantity to power gas mass quantity ratiocorresponding to said power gas.

Finally, the ignition advance correction can be determined according tothis stoichiometric ratio.

In particular, the present invention is a method of ignition advancecorrection of an internal-combustion engine according to the compositionvariation of the power gas, notably of the natural gas contained in atank and supplying the engine. A methane number relative to the naturalgas filling the tank is determined and an ignition advance correctionvalue is determined from the number. A correlation is establishedaccording to the invention between the methane number described above,an air/fuel ratio R for a stoichiometric combustion and the variation ofthe flow of gas required to reach a reference working point of theengine after the composition variation of this gas.

More precisely, the following operations are carried out:

a) determining the gas flow variation required to reach a referenceworking point of the engine after the composition variation of this gas;

b) calculating, from this flow rate variation, a methane number (IM)relative to the natural gas filling said tank;

c) deducing an ignition advance correction value from said number IM;and

d) correcting the ignition advance.

A method according to the invention calculates a methane number relativeto the natural gas filling the tank, then in correlating the methanenumber with an ignition advance correction value by means of apredetermined correspondence table, or in applying a given correction ifthe calculated number is below a predetermined value.

An air/fuel ratio R for stoichiometric combustion conditions depends onthe composition of the natural gas in the tank of the vehicle. It hasbeen found, within the scope of the present invention, that it ispossible to establish a correlation between this ratio and the methanenumber described above.

Furthermore, the engine control unit can permanently quantify, accordingto the present invention, the effect of a variation of ratio R after acomposition variation of the natural gas in the tank of a vehicle.

A suitable engine tuning can thus be found if the stoichiometricair/fuel ratio of the natural gas contained in the tank is permanentlyknown.

More specifically, the methane number is thus calculated according tothe air/fuel ratio (R) for a stoichiometric combustion. As alreadymentioned, ratio R depends on the chemical composition of the gas and itis therefore likely to change according to different tanks. Moregenerally, this ratio changes as soon as the composition of the gascontained in the tank changes, i.e. when the composition of the fuelinjected into the tank is different from that of the fuel previouslypresent in the tank.

More precisely, ratio (R) is determined wherein (R(1)) is the ratiodetermined before filling the tank and (R(2)) is the ratio determinedafter filling the tank, and a flow of gas Q(1) before filling isdetermined and a flow of gas Q(2) after filing is determined, for thesame working point of the engine. Ratio (R(1)) relative to the previoustankful and flow rate (Q(1)) are known because they are continuouslystored in the computer memory. Significant variations in the gascompositions between two successive tankfuls thus correspond to notablevariations between gas flows (Q(1)) and (Q(2)), thus between ratios(R(1)) and (R(2)). For the same reference working point of the engine(constant set air/fuel ratio and constant engine speed), it has beenfound that the stoichiometric Q air/Q fuel (gas) ratio corresponding tothe gas contained in the tank is inversely proportional to the flow rateof this gas.

It is therefore possible to calculate the ratio R(2) from the differenceof the inverse of the flow rates (1/Q(2))−(1/Q(1)) multiplied by a firstconstant Ki, a value to which ratio R(1), i.e. determined for theprevious tankful, is added.

It has also been found in the present invention that it is possible todetermine, in a surprisingly simple way, the methane number IM of thegas contained in the tank by multiplying ratio R(2) by a second constantK2 and by taking a third constant K3 away from the product. Values K2and K3 can for example be determined according to the nature of all thegases likely to be used in a given place or country.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features, details and advantages of the present invention will beclear from reading the description hereafter, given by way of nonlimitative example, with reference to the accompanying drawings wherein:

FIG. 1 is a flowchart of the method according to the invention; and

FIG. 2 gives a characteristic of different gases by means of their ratioR according to the calculated methane number IM.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The organization described hereafter provides an ignition advancecorrection factor C which produces optimum running conditions for theengine, without engine knocking, whatever the composition of the gassupplying the engine.

It is well-known that the very good behavior of natural gas regardingengine knocking contributes to reaching an optimum engine tuning. Thephysico-chemical parameter permitting this ability to be obtained is themethane number IM which depends on the composition of the gas.

This methane number can be estimated from an experimental correlationfor a series of reference gases, by taking account of the mostinfluential chemical species.

The method according to the invention comprises a particular calculationof methane number IM as explained hereafter in connection with FIG. 1.

The calculation can be initiated from known values R(0), Q(O) defined asstandards for each working point of the engine or reference point. Anengine working point can be conventionally defined by the rotating speedof the engine and by the load of the engine. This load can notably bedefined by the pressure prevailing in the inlet manifold, by thethrottle position,

After filling the vehicle with natural gas, the composition of the gascontained in the tank has probably varied for the reasons explainedabove.

When the vehicle is used thereafter, the computer stores the new flow ofgas required to reach the reference working point and to meet the setmixture strength.

It is in fact necessary to vary the flow of gas in order to reach theset mixture strength for this reference working point. This flowvariation leads to a variation of the air/fuel ratio (R) for thestoichiometric conditions. According to the present invention, and for agiven working point of the engine, the relationship exists:

R(1)=K 1/[Q(1)] and R(2)=K 1/[Q(2)].

Q(2) and Q(1) can be determined by measurement,

K1 being a known given constant,

R(1) being the ratio stored before filling,

R(2), which is the final ratio after filling which is deduced from thefollowing relationship:${R(2)} = {{R(1)} + {{K1} \times ( {\frac{1}{Q(2)} - \frac{1}{Q(1)}} )}}$

The methane number IM of the gas contained in the tank is thendetermined by means of the formula as follows:

IM=K 2*R(2)−K 3.

A corrective ignition advance value is thus applied according to thevalue of the calculated methane number IM.

By way of example, the line shown in FIG. 2 was obtained with K2=95 andK3=1535.

It appears that “H” type gases are scattered around the line andtherefore that there is a good correlation.

However, for some gases such as Netherland or Norway gases of type “B”,the calculated methane number is very far from the curve. A specificprocessing is therefore applied if the calculated value TM is less thanor equal to a certain predetermined value X.

Application of the aforementioned correlation formula would give a verylow IM number, of the order of 35, so that the corresponding advancereduction would be unsuitable for the engine. Furthermore, it has beenestablished during tests that the higher the proportion of inertmaterial in the natural gas, the lower the combustion velocities, allother things being equal. For these gases, the suitable correctiveaction thus consists in increasing the ignition advances in order tolimit the combustion lag in the cycle instead of reducing ignitionadvance. A fixed correction (C=K) is applied in these cases.

For the other gases, whose IM number is above the value X, a correctionC is applied according to a correspondence table C=f(IM) belonging toengine maps.

More generally, low ignition advances will be applied for low IM numbersand conversely.

Value X was selected equal to 60 in the tests carried out by theassignees. This value is purely illustrative and in no way limitative.

A particular embodiment of the method according to the invention isdescribed hereafter.

1) Waiting for the engine to reach the reference working point;

Remarks:

a—the reference point is the engine working point at which the strategydescribed will be applied;

b—the reference point is perfectly defined and without ambiguity by theair mass flow rate (Qair) and the engine speed;

c—several reference points can perfectly be considered for example inorder to carry out cross-checks according to strategies which remain tobe defined. However, a reference point must be selected according to thepossible repeatability level to be reached in order to guarantee thereliability of this strategy (beware of running without mixture strengthloop control);

2) Determination by measurement of the rotating speed (speed detector onring gear) and of the load (manifold pressure, throttle position, . . .) is systematically performed on standard engines and it is thereforepermanently available;

3) Determination of the air flow rate of the reference point, bycalculation (the cylinder capacity, the speed, the filling according tothe load, the thermodynamic conditions at the intake are known) or byreading an engine map if available;

4) From the mixture strength formula, at the same reference point, therelationship exists:

Qgas*RAFST=Qair*mixture strength=cte.

In this example, RAFST is the stoichiometric air/fuel mass ratio andTMTFP is the calculated methane number.

Storage of the initial conditions [1]:

Before filling the tank, [1], the computer has stored Qgas[1] andRAFST[1].

Determination of Qgas[2] after filling:

In order to reach the set mixture strength, the engine control willadjust the injection time Ti[2] from which it is possible to knowQgas[2}.

Test:

If Qgas[2]−Qgas[1}<epsilon, a small number to be defined, nothing isdone, otherwise continue.

Determination of RAFST[2]:

RAFST[1]*Qgas[1]=RAFST{2]*Qgas[2]

RAFST[2]=RAFST[1]*Qgas[1]/Qgas[2].

Determination of IMIFP[2]:

The following relation has been established for a natural gas poolselected as the reference:

IMIFP=K2*RAFST+K3

IMIFP[2]=K2*RAFST[2]+K3.

Test:

If calculated IMIFP[2] >60 (gas If calculated IMIFP[2] <60 (gas type“H”) type “B”) Application of the advance Application of the advancecorrection correction correction: f(IMIFP) to be defined for gas B withreal methane number - 85

The advance correction is adjusted according to IMIFP. These correctionsaccording to IMTFP may have been predetermined by means of tests duringengine tuning.

According to the invention, it is possible to use one or more referencepoints.

The reference point is preferably selected in a common engine operatingrange, for example at idle speed. Idle speed also has the advantage ofhaving well-adjusted air flow rates, which facilitates determination ofthe different values required for implementing the invention.

It is possible, according to the method, to measure the fuel variationsbetween two tankfuls, but this is also possible in relation tocharacteristics of an initial fuel. Similarly, the method according tothe invention can be implemented continuously or intermittently withoutwaiting for a new tank filling.

What is claimed is:
 1. A method of ignition advance correction of aninternal-combustion engine running on a power gas, comprising:determining at least one reference working point of the engine, avariation of a quantity linked with a flow rate variation of the powergas entering the engine and an ignition advance correction from avariation of ignition advance; and wherein the quantity linked with theflow rate variation of the power gas is the flow of the power gas; fromvariation of the flow of the power gas and from a mixture strength of afuel/gas mixture supplying the engine, a stoichiomentric air massquantity to power gas mass quantity ratio corresponding to the power gasis determined; and the ignition advance correction is determinedaccording to the ratio.
 2. A method as claimed in claim 1, wherein: theat least one reference working point of the engine corresponds to aworking point of the engine at idle speed.
 3. A method as claimed inclaim 1, wherein: the at least one reference working point is pluralreference points of the engine.
 4. A method as claimed in claim 2wherein: the at least one reference working point is plural referencepoints of the engine.
 5. A method of ignition advance correction of aninternal-combustion engine according to variation in composition ofnatural gas contained in a tank and supplied to the engine, comprising:a) determining a gas flow variation of the natural gas required to reacha reference working point of the engine after the variation of thecomposition of the natural gas; b) calculating, from the flow ratevariation, a methane number (IM) relative to the natural gas filling thetank; c) determining an ignition advance correction value from thenumber IM; and d) correcting the ignition advance.
 6. A method asclaimed in claim 5, wherein: the methane number (IM) is calculatedaccording to an air/fuel ratio (R) for a stoichiometric combustion.
 7. Amethod as claimed in claim 6, wherein for the ratio (R) wherein (R(1))is the ratio determined before filling the tank and (R(2)) is the ratiodetermined after filling the tank, and a flow of natural gas (Q(1))before filling the tank is determined and a flow of natural gas (Q(2))after filling the tank, for a same working point of the engine, isdetermined.
 8. A method as claimed in claim 7, wherein: (R(2)) iscalculated from a difference of an inverse of the flow rates(1/Q(2)−1/Q(1)) multiplied by a first constant K1, and a value to whichratio (R(1)), determined for a previous tankful is added.
 9. A method asclaimed in claim 8, wherein: the methane number (IM) is determined bymultiplying ratio (R(2)) by a second constant K2 to produce a productand by subtracting a third constant K3 away from the product.
 10. Amethod as claimed in claim 9, wherein: the values K2 and K3 aredetermined according to a characteristic of gases available in place orcountry.
 11. A method as claimed in claim 5, wherein: the at least onereference working point of the engine corresponds to a working point ofthe engine at idle speed.
 12. A method as claimed in claim 6, wherein:the at least one reference working point of the engine corresponds to aworking point of the engine at idle speed.
 13. A method as claimed inclaim 7, wherein: the at least one reference working point of the enginecorresponds to a working point of the engine at idle speed.
 14. A methodas claimed in claim 8, wherein: the at least one reference working pointof the engine corresponds to a working point of the engine at idlespeed.
 15. A method as claimed in claim 9 wherein: the at least onereference working point of the engine corresponds to a working point ofthe engine at idle speed.
 16. A method as claimed in claim 10 wherein:the at least one reference working point of the engine corresponds to aworking point of the engine at idle speed.
 17. A method as claimed inclaim 5, wherein: the at least one reference working point is pluralreference points of the engine.
 18. A method as claimed in claim 6,wherein: the at least one reference working point is plural referencepoints of the engine.
 19. A method as claimed in claim 7, wherein: theat least one reference working point is plural reference points of theengine.
 20. A method as claimed in claim 8, wherein: the at least onereference working point is plural reference points of the engine.
 21. Amethod as claimed in claim 9 wherein: the at least one reference workingpoint is plural reference points of the engine.
 22. A method as claimedin claim 10 wherein: the at least one reference working point is pluralreference points of the engine.